It's normal to fib about some things. "So sorry we won't make the potluck — can't find a sitter." You promise your mother you'll call. But the one person you should never, ever lie to is your doctor. Yet we do. All the time. A national survey recently revealed that 52 percent of women routinely stretch the truth when they talk to their doctors — exaggerating how much exercise they get, lowballing how much they smoke or drink, even hiding sexual behavior. We lie, mainly, because we know we're not being as dedicated as we should and we don't want to feel judged or endure a lecture we've heard before. (Hey, we're not stupid. Lazy, perhaps, but not stupid!)

Other lies just...slip out. It can be hard in a short visit to bring up behavior we might be ashamed of (even if there's no reason to be — docs have seen and heard it all before, and worse). We figure, what's the harm in omitting a few minor details — like that STD we had in college, or that one time we forgot to take our birth control?

In fact, more than a quarter of the women in the survey didn't believe their lies were a big deal. But lying to the one person who really needs to know the truth — and is bound by doctor/patient privilege and federal law to keep that info private — can be a very big deal. When you tell even a fib, your doctor can't diagnose you correctly, which wastes your time and money and may keep her from giving treatment that could save your life. So the next time you're tempted to make like Pinocchio with one of the following falsehoods, here's the truth about why you should tell nothing but.

"When I was in practice, I heard this lie every day," laughs Paula Jones, D.D.S., now president of the Academy of General Dentistry. "I'd ask, 'How often?'" And the truth would start to come out. "They'd say, 'Oh, a couple of times a week' or 'I only do this one tooth where food gets caught.'"

WHY YOU SHOULD COME CLEAN: Neglecting to floss leads directly to tooth decay, gum inflammation, and gum disease — and a growing body of research suggests that gum disease may contribute to cardiovascular disease. Some studies also suggest a link between gum disease and a life-threatening pregnancy complication called preeclampsia. If you cop to being a non-flosser, your dentist can make doubly sure to watch for and help you prevent these dangerous conditions.

THE LIE: "I've never had an STD."

"Many women are embarrassed about having had a sexually transmitted disease," says Dimino. Jessica, 37, never let on that she'd had chlamydia and HPV in college because she was afraid she'd be denied health insurance when setting up her own business. But she also believed it wasn't anyone else's affair. "My feeling was: That was then; it's taken care of, so they don't need to know," says the Littleton, CO, Web designer.

WHY YOU SHOULD COME CLEAN: The ghosts of STDs past can come back to haunt you later. Hiding that you had HPV may put you at higher risk for cervical cancer if your gyno doesn't think you need annual Pap tests. Keeping quiet may also put you at risk for pregnancy complications. For instance, if your doctor knows you've had gonorrhea or chlamydia — especially if you also developed pelvic inflammatory disease — she'll watch more carefully for ectopic pregnancy, since both can scar the fallopian tubes, preventing a fertilized egg from reaching the uterus. And telling your ob that you have genital herpes when you're pregnant may help you avoid a predelivery flare-up — and a C-section as a result. "If you tell me you have it, even if your last flare-up was ages ago, I can put you on medication to suppress an outbreak before delivery," says Dimino.

Doctors have an unspoken rule: Whatever you tell them you drink, they double it. "Lots of women claim, 'I drink once a week,'" says Rakhi Dimino, M.D., an ob/gyn at the Woman's Hospital of Texas in Houston. "But then they drink six or seven cocktails in an evening."

"I always lie about drinking," admits Amanda, 33, an entertainment coordinator from Orlando, FL. "My doctor prescribed me the antidepressant Zoloft and told me not to drink any alcohol while taking it. Then I went to a wine tasting and drank anyway." That night, she awoke in a cold sweat with heart palpitations. "At the emergency room, the doctor asked me which drugs I was on. When I said Zoloft, he asked if I'd taken any drugs or drunk any alcohol. I flat-out lied and said that I'd had not a drop."

The ER staff tried to sleuth out what else could be making Amanda ill; then her blood alcohol test came back. "The doctor said, 'The test shows you've been drinking, and this is a common reaction with Zoloft and alcohol.' He told me that I would have saved a lot of time and money if I'd been honest. I didn't say a thing. I just wanted to get out of there with my tail between my legs."

WHY YOU SHOULD COME CLEAN: Drug and alcohol interactions are very common. If your doctor knows that you imbibe, even a little, she can prescribe meds that won't mix badly with a glass of wine. She can also counsel you on alcohol's risks for women. For starters, having one to two drinks a day can raise your risk for breast cancer; heavy consumption is linked to liver disease, brain damage, and stroke and can put you at risk for assault and car accidents.

Concealing what you drink may signal another disease: "Denial is part of addiction," says Brenda Iliff, clinical director of Hazelden Women's Recovery Center in Center City, MN.

THE LIE: "I'm monogamous."

Cheryl, 48, went to see her gyno for what she thought was a yeast infection — and was shocked to learn she actually had trichomoniasis, an STD. She didn't want to admit she was juggling four guys, so when the doctor asked how many partners she had, "I said one, of course," recalls the accountant from Knoxville, TN. The doctor gave Cheryl enough medication for her and her partner. But Cheryl kept seeing the other guys too. "I went back for my checkup, and my gyno says, 'You still have this. You haven't told me the whole truth, have you?' I said, 'Yes, I have.' She didn't want to call me a liar, but she said, 'You have to treat everyone you're seeing, or quit seeing the ones you don't want and treat the one you do. Otherwise, you'll never get rid of this.'" Cheryl dumped the other guys and continued treatment with her main man. "But afterward, I changed doctors," she admits. "I couldn't face her anymore."

WHY YOU SHOULD COME CLEAN: Your doctor doesn't ask about your sex life to judge your morals. What does concern her is that sleeping with more than one person may increase your risk for STDs. Delayed STD treatment can mean a more entrenched pelvic infection, fertility problems — even cervical cancer. "If your gyno knows you have several partners, she may recommend you have an annual Pap test and get screened more frequently for STDs," says Dimino.

"I have patients who swear they're exercising and sticking to the calorie count," says Bonnie Davis, an advanced registered nurse practitioner in Largo, FL, who helps administer a weight-management program. "Yet they've put on 5 pounds while taking an appetite suppressant three times a day. That's impossible."

Meredith, 26, sticks with the purposely vague "sometimes" when asked how often she exercises. "I wouldn't feel right saying 'regularly,'" says the writer from Forest Hills, NY. "But when I say, 'Sometimes,' I consider that I walk to the subway every day, and if I'm not wearing heels, I walk fairly briskly." In other words, she's not lying outright — just bending the truth enough to spare herself the inevitable lecture. "I know that losing 10 pounds could lower my risk for heart disease and diabetes, and diabetes does run in my family," Meredith says. "But I don't want to hear it. I'd rather doctors think that I take it seriously than give them the opportunity to tell me what I already know but still am not paying attention to."

WHY YOU SHOULD COME CLEAN: If your blood pressure and cholesterol are high or you're borderline diabetic — all factors that can boost your risk for cardiovascular disease — diet and exercise can help, which is why your doctor asks about them. But if you're not really making either lifestyle change and your numbers don't get lower, your doc may put you through a battery of pricey medical tests and/or prescribe a range of medications to lower them for you. And while taking a pill may sound easier than counting calories and hitting the gym, it actually "opens up a Pandora's box of inconvenience," says Nora Tossounian, M.D., an internist at the Women's Health Center at Hackensack University Medical Center in New Jersey. Start with the nuisance of remembering to take medication once or twice a day; add to that the high cost of those meds. Then there are the side effects: muscle aches on statins; bloating, cramping, and diarrhea on diabetes medications; a plunging sex drive with certain blood pressure drugs. The truth hurts less.

THE LIE: "I don't smoke."

When Pamela Douglas, M.D., a cardiologist at Duke University Medical Center in Durham, NC, asks women if they smoke, she often hears a little moment of hesitation before they say no. "They believe you need to smoke two packs a day to be at risk," she explains. "They say they don't smoke them all the way down or they're not really inhaling. If they've only been smoking a year or two or they don't smoke every day, they think they're not really smokers."

WHY YOU SHOULD COME CLEAN: Reality check: If you light up, even if it's only one on the weekends, even if you just bum a drag from your friend, you smoke. Beyond an increased risk for sinus and upper respiratory infections, emphysema, cardiovascular disease, and, yes, lung cancer, lighting up — even occasionally — raises your risk for blood clots and stroke if you're also using hormone-based contraceptives (pills, patches, rings). "If there's a pause when I ask them if they smoke and they say, 'No' or 'Maybe once a month,' I'm hesitant to give that woman a prescription for the Pill" to help clear up her skin, says David Bank, M.D., medical director of the Center for Dermatology, Cosmetic and Laser Surgery in Mount Kisco, NY. Tell the truth so you and your doc can figure out a safe option together.

"Along with 'I'm not tan — this is my natural skin color,' this one's at the top of the list of lies we hear all day," chuckles Bank. "We ask every patient whether they use sunscreen every day, and about 10 percent to 20 percent of the responses we get are false or exaggerated."

WHY YOU SHOULD COME CLEAN: Skin cancer is the most common cancer in the United States; since 1980, the rate of melanoma (the most serious form of skin cancer) has jumped by 50 percent for women between the ages of 15 and 39, according to new research from the National Cancer Institute. If you admit you're not slathering on sunscreen (SPF 30 or higher, applied liberally to exposed areas), your doctor may schedule more frequent screenings — this way, if skin cancer does develop, it'll be caught early. Being truthful about sunscreen use can also help him decide whether to prescribe certain meds, like Retin-A for acne and wrinkles, that can make you more sensitive to the sun.

THE LIE: "I'm taking my medication the way you prescribed it."

Alicia, 31, often used her asthma inhaler up to five times a day, despite her doctor's warnings. When he noted her trembling hands and pallid face, "I swore I wasn't abusing it, because I was afraid he'd take it away," says the Orlando, FL, day-care teacher. "I didn't care about the risks as long as I could breathe."

WHY YOU SHOULD COME CLEAN: If you tell your doctor your medication isn't working, or has side effects, he can find one that suits you better (which is what Alicia's new doc eventually did). But if you don't use it correctly, you could end up even sicker. Take a typically misused drug like a routine antibiotic: "If you lie and say you finished your antibiotics, but you're still sick, the doctor will assume the first drug didn't work," says Maurice A. Ramirez, D.O., Ph.D., an emergency-room physician at Florida Hospital-Flagler Division in Palm Coast. "So he'll change the antibiotic. Meanwhile, the bacteria become resistant to the drug we normally use, and they crank along unimpeded, and you can go from a bladder infection to a kidney inflection to a blood infection."

This one's told so often, doctors say, that they always dig deeper when they hear it. Sometimes it's an honest mistake: "People don't classify over-the-counter drugs like Tylenol, herbal supplements, or vitamins as medication," says Gillian Stephens, M.D., an assistant professor of community and family medicine at Saint Louis University in St. Louis. "But they are." Women also keep mum, though, when they've "borrowed" an Ambien from a friend or bought Hoodia online to lose weight.

WHY YOU SHOULD COME CLEAN: Your doctor may adjust your dosage of a drug if he knows you're taking something else that could alter its effects. "Hearing that someone's taking ibuprofen, which is a weak blood thinner, is useful if you're adding other thinners," says John H. Alexander, M.D., a cardiologist at Duke University Medical Center. Not to mention, what he doesn't know could kill you. It's not unusual for doctors to realize that you've taken something you didn't tell them about only after they've given you another medication — and the combination has caused you to stop breathing, have a seizure, or go into cardiac arrest.

THE LIE: "I always use birth control."

"The lie I hear day after day is, 'The condom broke,'" says Millicent Comrie, M.D., vice chair of obstetrics and gynecology at Long Island College Hospital in Brooklyn, NY. "Women often say that when they get pregnant by accident and don't want to admit that they didn't use a condom." Sound familiar? How about this: "Women fudge the truth about whether they put their diaphragms in or took them out and whether they take their pills every day at the same time," says Bruce Rosenzweig, M.D., director of urogynecology at Rush University Medical Center in Chicago. "They're embarrassed because they know they should be able to handle these things."

WHY YOU SHOULD COME CLEAN: There are tons of contraception options out there these days. If you fess up to messing up with birth control, your gyno can suggest one that might suit you better. "If you hate to swallow pills, you have oily skin and the patch keeps falling off, or the ring comes out when your partner pulls out, I can give you something else," says Rosenzweig. "But if you don't tell me, I can't make the appropriate recommendations."

Coming clean with your doctor only stands to help you in the long run, as difficult as it may be in the moment. So answer her questions honestly — and even bring up issues that she may be forgetting to ask about. When it comes to your health, there's no such thing as TMI.